Allowed and Forbidden TransitionsAllowed and Forbidden Transitions

Only a fraction of all possible transitions are observed.Only a fraction of all possible transitions are observed.Allowed transitionsAllowed transitions

-high probability, high intensity, electric dipole -high probability, high intensity, electric dipole interactioninteractionForbidden transitionsForbidden transitions

-low probability, weak intensity, non-electric -low probability, weak intensity, non-electric dipole interactiondipole interaction

Selection rules for allowed transitions:Selection rules for allowed transitions:* The parity of the upper and lower level must be * The parity of the upper and lower level must be different. (The parity is even if different. (The parity is even if llii is even. The is even. The parity is parity is

odd if odd if llii is odd.) is odd.)

* * ll = ±1 = ±1* * JJ = 0 or ±1, but = 0 or ±1, but JJ = 0 to = 0 to JJ = 0 is forbidden. = 0 is forbidden.

Additional Splitting EffectsAdditional Splitting Effects

•Hyperfine splittingHyperfine splitting due to magnetic coupling of spin and orbital due to magnetic coupling of spin and orbital motion of electrons with the nuclear spin.motion of electrons with the nuclear spin.

•Isotope shift. Sufficient to determine isotope ratios.Isotope shift. Sufficient to determine isotope ratios.

•Splitting in an electric field (Stark effect): Relevant for arc and Splitting in an electric field (Stark effect): Relevant for arc and spark techniques.spark techniques.

•Splitting in a magnetic field (Zeeman effect):Splitting in a magnetic field (Zeeman effect):

* In absence of a magnetic field, states that differ * In absence of a magnetic field, states that differ only by their only by their MMJJ values are degenerate, i.e., they values are degenerate, i.e., they have have equivalent energies.equivalent energies.

* In presence of a magnetic field, this is not true * In presence of a magnetic field, this is not true anymore.anymore.

* Can be used for background correction.* Can be used for background correction.

Pretsch/Buhlmann/Affolter/Badertscher, Pretsch/Buhlmann/Affolter/Badertscher, Structure Determination of Organic CompoundsStructure Determination of Organic Compounds

Pretsch/Buhlmann/Affolter/Badertscher,Pretsch/Buhlmann/Affolter/Badertscher,Structure Determination of Structure Determination of Organic CompoundsOrganic Compounds

Stark SplittingStark Splitting

www.wikipedia.orgwww.wikipedia.org

For H:For H:split split E E

For others:For others:split split (E) (E)22

Zeeman SplittingZeeman Splitting

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

MMJJ – Resultant total – Resultant total

magnetic quantummagnetic quantumnumbernumber

MMJJ = J, J-1, …, -J = J, J-1, …, -J

2J +1 possible values2J +1 possible values

NormalNormal AnomalousAnomalous

Sample Introduction and AtomizationSample Introduction and Atomization

Atomization:Atomization:Convert solution Convert solution → vapor-phase free atoms→ vapor-phase free atoms

Measurements usually made in hot gas or enclosed furnace:Measurements usually made in hot gas or enclosed furnace:

•flamesflames•plasmasplasmas•electrical discharges (arcs, sparks)electrical discharges (arcs, sparks)•heated furnacesheated furnaces

Free Free AtomsAtoms

Free Free AtomsAtoms

IonsIonsIonsIonsMole-Mole-culescules

Mole-Mole-culescules

NebulizationNebulization

DesolvationDesolvation

VolitalizationVolitalization

Adapted from Ingle and CrouchAdapted from Ingle and Crouch

Atomic Emission Spectroscopy (AES)Atomic Emission Spectroscopy (AES)

See also: Fundamental reviews in See also: Fundamental reviews in Analytical ChemistryAnalytical Chemistry e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal. Anal. Chem. Chem. 20022002, , 7474, 2691-2712 (“Atomic Spectroscopy”), 2691-2712 (“Atomic Spectroscopy”)

•Beginning 19th century: alcohol flame (Brewster, Herschel, Talbot, Beginning 19th century: alcohol flame (Brewster, Herschel, Talbot, Foucault)Foucault)

•mid 1800s: Discovery of Cs, Tl, In, Ga by atomic spectroscopy mid 1800s: Discovery of Cs, Tl, In, Ga by atomic spectroscopy (Bunsen, Kirchhoff)(Bunsen, Kirchhoff)

•1877: Gouy designs pneumatic nebulizer1877: Gouy designs pneumatic nebulizer

•1920s: Arcs and sparks used for AES1920s: Arcs and sparks used for AES

•1930s: First commercial AES spectrometer (Siemens-Zeiss)1930s: First commercial AES spectrometer (Siemens-Zeiss)

•1960s: Plasma sources (commercial in 1970s)1960s: Plasma sources (commercial in 1970s)

Atomic Emission Spectroscopy (AES)Atomic Emission Spectroscopy (AES)2S1/2

22DD3/2, 5/23/2, 5/222PP3/23/2

22PP1/21/222SS1/21/2

At RT, nearly allAt RT, nearly allelectrons in 3selectrons in 3sorbitalorbital

Excite with flame, Excite with flame, electric arc, or electric arc, or sparkspark

Common electronicCommon electronictransitionstransitions

http://raptor.physics.wisc.edu/data/e_sodium.gifhttp://raptor.physics.wisc.edu/data/e_sodium.gif

Example AE SpectraExample AE Spectra

http://www.technology.niagarac.on.ca/lasers/Chapter2.htmlhttp://www.technology.niagarac.on.ca/lasers/Chapter2.html

HH22

HgHg

HeHe

An Ideal AES SourceAn Ideal AES Source

1. complete atomization of all elements2. controllable excitation energy3. sufficient excitation energy to excite all elements4. inert chemical environment5. no background6. accepts solutions, gases, or solids7. tolerant to various solution conditions and solvents8. simultaneous multi-element analysis9. reproducible atomization and excitation conditions10. accurate and precise analytical results11. inexpensive to maintain12. ease of operation

Flame AESFlame AES

•Background signals due to flame fuel and oxidants – line spectra:Background signals due to flame fuel and oxidants – line spectra:

•OHOH•• 281.1, 306.4, 342.8 nm 281.1, 306.4, 342.8 nm from O + Hfrom O + H22 H + OH H + OH

H + OH + O22 O + OH O + OH

•OO22 250, 400 nm250, 400 nm

•CHCH 431.5, 390.0, 314.3 nm431.5, 390.0, 314.3 nm•COCO bands between 205 to 245 nmbands between 205 to 245 nm•CN, CCN, C22, CH, NH bands between 300 to 700 nm, CH, NH bands between 300 to 700 nm

Unlike bands of atomic origin, these molecular bands are fairly broad.Unlike bands of atomic origin, these molecular bands are fairly broad.

•Continuum emission from recombination reactionsContinuum emission from recombination reactions e.g. H + OH e.g. H + OH H H22O + hO + h CO + O CO + O CO CO22 + h + h

Flames used in AES nowadays only for few elements. Cheap but Flames used in AES nowadays only for few elements. Cheap but limited. {Flame AES often replaced by flame AAS.}limited. {Flame AES often replaced by flame AAS.}

Ingle and CrouchIngle and Crouch

Inductively Coupled Plasma AESInductively Coupled Plasma AES•Spectral interference more likely for plasma than for flame due to larger Spectral interference more likely for plasma than for flame due to larger population of energetically higher states.population of energetically higher states.

•Modern ICP power: 1–5 kW (4 to 50 MHz)Modern ICP power: 1–5 kW (4 to 50 MHz)

•4000 to 10,000 K: Very few molecules4000 to 10,000 K: Very few molecules

•Long residence time (2–3 ms)Long residence time (2–3 ms) results in high desolvationresults in high desolvation and volatilization rateand volatilization rate

•High electron density suppresses High electron density suppresses ionization interference effectsionization interference effects

•Background: Ar atomic lines and,Background: Ar atomic lines and, in hottest plasma region, in hottest plasma region, Bremsstrahlung (continuum radiationBremsstrahlung (continuum radiation from slowing of charged particles) from slowing of charged particles)

•Price > $ 50 kPrice > $ 50 k

•Operating cost relatively high dueOperating cost relatively high due to Ar cost (10–15 mL/min) andto Ar cost (10–15 mL/min) and training.training. www.wikipedia.org,www.wikipedia.org, Ingle and CrouchIngle and Crouch

Microwave Plasma AESMicrowave Plasma AES•Power 25 to 1000 W Power 25 to 1000 W (ICP 1000–2000 W)(ICP 1000–2000 W)

•Frequency 2450 MHz Frequency 2450 MHz (ICP 4 to 50 MHz)(ICP 4 to 50 MHz)

•Argon, helium or nitrogenArgon, helium or nitrogen

•Temperature estimated to be 2000 - 3000 KTemperature estimated to be 2000 - 3000 K

•Low temperature causes problems with liquidsLow temperature causes problems with liquids

•Useful for gases: Useful for gases: GC–microwave plasma AESGC–microwave plasma AES

Arcs and SparksArcs and Sparks•Arc = Arc = An electrical discharge between two or more An electrical discharge between two or more conducting electrodes (1-30 A)conducting electrodes (1-30 A)•Spark = Spark = An intermittent high-voltage discharge (few An intermittent high-voltage discharge (few sec)sec)

•Limited to qualitative and semi-quantitative use (arc Limited to qualitative and semi-quantitative use (arc flicker)flicker)•Particularly useful for solid samples (pressed into Particularly useful for solid samples (pressed into electrode)electrode)•The burn takes > 20 sec (need multichannel detector)The burn takes > 20 sec (need multichannel detector)

Ingle and CrouchIngle and Crouch